1. Field of the Invention
The present invention is directed in general to field of information processing. In one aspect, the present invention relates to a system and method for precoding feedback in MIMO communication systems.
2. Description of the Related Art
Wireless communication systems transmit and receive signals within a designated electromagnetic frequency spectrum, but the capacity of the electromagnetic frequency spectrum is limited. As the demand for wireless communication systems continues to expand, there are increasing challenges to improve spectrum usage efficiency. To improve the communication capacity of the systems while reducing the sensitivity of the systems to noise and interference and limiting the power of the transmissions, a number of wireless communication techniques have been proposed, such as Multiple Input Multiple Output (MIMO), which is a transmission method involving multiple transmit antennas and multiple receive antennas. For example, space division multiple access (SDMA) systems can be implemented as closed-loop systems to improve spectrum usage efficiency. SDMA has recently emerged as a popular technique for the next generation communication systems. SDMA based methods have been adopted in several current emerging standards such as IEEE 802.16 and the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) platform.
FIG. 1 depicts a MIMO wireless communication system 100 in which a transmitter 102 having a first antenna array 106 communicates with receiver 104 having a second antenna array 108, where each antenna array includes one or more antennas. The communication system 100 may be any type of wireless communication system, including but not limited to a MIMO system, SDMA system, CDMA system, OFDMA system, OFDM system, etc. In the communication system 100, the transmitter 102 may act as a base station, while the receiver 104 acts as a subscriber station, which can be virtually any type of wireless one-way or two-way communication device such as a cellular telephone, wireless equipped computer system, and wireless personal digital assistant. Of course, the receiver/subscriber station 104 can also transmits signals which are received by the transmitter/base station 102. The signals communicated between transmitter 102 and receiver 104 can include voice, data, electronic mail, video, and other data, voice, and video signals. In operation, the transmitter 102 transmits a signal data stream (e.g., signal s1) through one or more antennas 106 and over a channel H1 to a receiver 104, which combines the received signal from one or more receive antennas 108 to reconstruct the transmitted data. To transmit the signal s1, the transmitter 102 prepares a transmission signal, represented by the vector x1, for the signal s1. (Note: lower case bold variables indicate vectors and upper case BOLD variables indicate matrices). The transmission signal vector x1 is transmitted via a channel represented by a channel matrix H1, and is received at the receiver 104 as a receive signal vector y1=H1x1+n1 (where n represents co-channel interference or noise). The channel matrix H1 represents a channel gain between the transmitter antenna array 106 and the subscriber station antenna array 108. Thus, the channel matrix H1 can be represented by a k×N matrix of complex coefficients, where N is the number of antennas in the base station antenna array 106 and k is the number of antennas in the subscriber station antenna array 108. The value of k can be unique for each subscriber station. As will be appreciated, the channel matrix H1 can instead be represented by a N×k matrix of complex coefficients, in which case the matrix manipulation algorithms are adjusted accordingly so that, for example, the right singular vector calculation on a k×N channel matrix becomes a left singular vector calculation on a N×k channel matrix. The coefficients of the channel matrix H1 depend, at least in part, on the transmission characteristics of the medium, such as air, through which a signal is transmitted. A variety of methods may be used at the receiver to determine the channel matrix H1 coefficients, such as transmitting a known pilot signal to a receiver so that the receiver, knowing the pilot signal, can estimate the coefficients of the channel matrix H1 using well-known pilot estimation techniques. Alternatively, when the channel between the transmitter and receiver are reciprocal in both directions, the actual channel matrix H1 is known to the receiver and may also be known to the transmitter.
While the benefits of MIMO are realizable when the receiver 104 alone knows the communication channel, these benefits are further enhanced in “closed-loop” MIMO systems when the transmitter 102 has some level of knowledge concerning the channel response between each transmit antenna element and each receive antenna element. Precoding systems provide an example application of closed-loop systems which exploit channel-side information at the transmitter (“CSIT”). With precoding systems, CSIT can be used with a variety of communication techniques to operate on the transmit signal before transmitting from the transmit antenna array 106. For example, precoding techniques can provide a multi-mode beamformer function to optimally match the input signal on one side to the channel on the other side. In situations where channel conditions are unstable or unknown, open loop MIMO techniques such as spatial multiplexing can be used. However, when the channel conditions can be provided to the transmitter, closed loop MIMO methods such as precoding can be used. Precoding techniques may be used to decouple the transmit signal into orthogonal spatial stream/beams, and additionally may be used to send more power along the beams where the channel is strong, but less or no power along the weak, thus enhancing system performance by improving data rates and link reliability. In addition to multi-stream transmission and power allocation techniques, adaptive modulation and coding (AMC) techniques can use CSIT to operate on the transmit signal before transmission on the array 106.
Conventional precoded MIMO systems may obtain full broadband channel knowledge at the transmitter 102 by using uplink sounding techniques (e.g., with Time Division Duplexing (TDD) systems). Alternatively, channel feedback techniques can be used with MIMO systems (e.g., with TDD or Frequency Division Duplexing (FDD) systems) to feed back channel information to the transmitter 102. One way of implementing precoding over a low rate feedback channel is to use codebook-based precoding to reduce the amount of feedback as compared to full channel feedback. However, such precoding feedback can introduce delay when transmission-related decisions (such as whether to use spatial multiplexing or beamforming, i.e., multi-stream or single stream transmission, or whether to use open loop or closed loop transmission) are made at the transmitter only after receiving the channel information feedback signal and/or when the transmission-related decisions are made in the upper software layers (e.g., the MAC layer) of the transmitter 102. In addition, the quantization techniques used in existing codebook systems to compress the channel feedback information can introduce errors in the feedback signal. Moreover, the limited feedback resources require that any practical system be designed to have a low feedback rate, and existing codebook systems can have unacceptably high feedback data rates. This may be illustrated with the example of a MIMO system 100 having a four-antenna transmit array 106 and a two-antenna receive array 108, where a unitary precoding matrix W and a non-unitary power allocation precoding matrix D (not shown) are applied at the transmitter 102 to precode the transmit signal vector s1 (such that x1=WD s1). With such a system that is used to provide both spatial multiplexing and beamforming, the power allocation values d1, d2 of the non-unitary precoding matrix
  D  =      [                                        d            1                                    0                                      0                                      d            2                                ]  are each selected from the set of {0, 1, ½}, and each power allocation value is represented by a two-bit feedback value to choose from the set of three possible values. The two-bit feedback allows each of the power allocation values to be specified for spatial multiplexing—(d1, d2)=(½, ½)—and also separately for each instance of beamforming—e.g., (d1, d2)=(1, 0) and (d1, d2)=(0, 1). However, using two or more feedback bits to specify just the d1 and d2 values, not to mention additional precoding feedback bits for W, can impair feedback performance.
Accordingly, an efficient feedback methodology is needed to provide precoding feedback to the transmitter using a codebook to reduce the size of the feedback signal while sustaining a minimal loss in link performance. There is also a need for an improved feedback system which avoids degrading uplink performance and reduces long feedback delay. In addition, there is a need for a system and methodology for reducing the average precoder feedback rate to reduce uplink performance loss and feedback delay. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for purposes of promoting and improving clarity and understanding. Further, where considered appropriate, reference numerals have been repeated among the drawings to represent corresponding or analogous elements.